The present invention relates to a process for melting/sintering powder particles for the layer-by-layer production of three-dimensional objects.
A task frequently encountered in very recent times is the rapid production of prototypes. Processes to quickly prepare prototype objects are described as rapid prototyping/rapid manufacturing, or else additive fabrication processes. Particularly suitable processes are those which are based on pulverulent materials and in which the desired structures are produced layer-by-layer through selective melting and solidification. Supportive structures for overhangs and undercuts may be omitted with the use of pulverulent materials because sufficient supportive effect is provided from the construction-field plane surrounding the molten regions. The downstream operation of removing supports is also omitted. The processes may also suitable for short-run production. The construction chamber temperature may be selected in such a way that the structures produced layer-by-layer do not warp during the construction process.
A process which has particularly good suitability for the purpose of rapid prototyping is selective laser sintering (SLS). In this process, plastics powders are selectively briefly irradiated by a laser beam in a chamber, and the powder particles impacted by the laser beam therefore melt. The molten particles coalesce and rapidly resolidify to give a solid mass. This process can provide easy and rapid production of three-dimensional products by repeated irradiation of a succession of freshly applied layers.
The process of laser sintering (rapid prototyping) for producing mouldings made of pulverulent polymers is described in detail in U.S. Pat. No. 6,136,948 and WO 96/06881 (both DTM Corporation). A wide variety of polymers and copolymers are claimed for this application, examples being polyacetate, polypropylene, polyethylene, ionomers and polyamide.
The coordinate system of this type of apparatus is usually defined such that the x coordinate runs within the construction-field plane parallel to the traverse of the powder-application apparatus. The location of the y coordinate is within the construction-field plane, perpendicular to the traverse of the powder-application apparatus. The z coordinate is perpendicular to the construction-field plane.
In conventionally known processes and apparatuses for the prototyping, the area requiring selective hardening is traversed in straight-line sections, line by line (x-direction) or column by column (y-direction) by a beam of electromagnetic radiation. In the present application the beam of electromagnetic radiation may be referred to as an energy beam. DE 10233389 A1 moreover discloses a straight-line traverse within particular angles within the x,y-plane.
According to the methods conventionally known, irradiation lines in the form of parallel straight-line sections are bounded by the external edge of the area requiring hardening. The distance between the lines or columns may be adjustable. The design of the energy beam is usually such that, as far as possible, the focal point of the energy beam is within the construction-field plane of the pulverulant material. In order to improve the surface of the object to be produced, the profile of the area requiring hardening is often additionally traversed by the energy beam. The time required by the energy beam to traverse the region is determined mainly by the traverse speed of the energy beam and the distance between the individual lines or columns. An obvious way of accelerating the process is therefore to increase the traverse speed of the energy beam and/or the distance between the lines or columns. The traverse speed of the energy beam cannot be increased without restriction because if acceleration effects or retardation effects are excessive, it becomes impossible to position the scanner system mirrors with precision. The objects produced then have uneven areas at the edges. Nor can the distance between the radiation lines be increased without restriction because, starting at a particular distance, the bonding between the individual irradiation lines becomes inadequate. The mechanical properties of the objects produced are then impaired.